Caspases have been well studied in the context of cell death. We are among the first to describe non-apoptotic functions for caspases. We published that caspases influence phagosome lysosome fusion. This application extends our findings by addressing the molecular mechanism by which caspase-4 and -5 promote vesicular trafficking in human macrophages. Legionella is a bacterium that causes Legionnaires' disease, a human illness characterized by severe pneumonia with fatality rates up to 30%. Legionella infects those above 50 years and the immunocompromised. There is no vaccine and prophylactic antibiotics do not protect the susceptible population. Therefore, understanding why humans are susceptible will help design new strategies for prevention and treatment in the future. We found that in human macrophages, the Legionella-containing vacuole does not fuse with the lysosome and instead the bacteria survives and replicates. Typically, precise actin polymerization and depolymerization are required for proper vesicular trafficking and phagosome lysosome fusion. We hypothesize that Legionella disrupts the function of the actin polymerization machinery to prevent lysosomal fusion and to establish infection in human macrophages. Therefore, this proposal will provide a molecular framework to understand the mechanism by which caspase-4 and -5 contribute to the innate immune response. We will identify downstream effector molecules that can be targeted with therapeutics in the future to promote Legionella clearance. We have the expertise and availability of human cells, which is a rare combination to pursue this novel project.